The present invention relates generally to devices for testing materials. More specifically, the present invention relates to an improved apparatus for a more reliable and accurate manner for measuring the bending stiffness of flexible materials such as synthetic elastomers and other rubber-like materials. The apparatus is adopted for use in a tension-compression testing device such as an "Instron" force/deflection tester, sometimes referred to herein for simplicity merely as a "tensiometer".
The test fixture of the invention includes a yoke portion positioning a pair of identical, material-engaging support units and an anvil portion including a single, material engaging unit forming its "nose" portion. In use, a sheet of flexible material is placed in spanwise relation above the support units of the yoke with the sheet extending under the nose of the anvil. When the yoke and anvil are respectively attached to the relatively movable elements of a conventional tensiometer, the bending stiffness of a particular material specimen can be measured. The provision of rotatable, specimen-engaging supports allows the material specimen to advance smoothly over the support rollers as it deflects under a pull from the nose portion; the rolling action prevents the specimen from binding or grabbing one or both of its support elements during measurement of bending stiffness.
By way of background, various devices and methods are known for testing the physical properties of a wide range of materials. Today, wide ranges of materials are tested for structural strength in various ways, and the test results are then used to classify the materials as to a particular property. Properties such as bulk modulus, compressive strength, tensile strength, shear strength, stiffness in bending, elasticity and other properties are routinely tested for.
Many structural materials may be broadly characterized as being low deformation materials. These low deformation materials include metals and hard plastics. These are considered "low deformation" because they show relatively slight deformation under a given force. Devices for testing these low deformation materials have typically included attachments for bending or flexure testing, using a compression load cell in a conventional tensiometer. Such structural and semi-structural materials include metals, alloys, hard plastics, chip board, particle board and the like. Tests of these materials commonly use a three- or four-point compression loading method to determine stress/strain values throughout a range appropriate for the materials tested. The test fixtures work well and give accurate, repeatable readings with these materials.
Conventional test fixtures have typically been unsatisfactory for certain kinds of testing done on softer materials, such as rubber-like materials and flexible polymers. Where only the "stretchability" or extension/contraction, hysteresis and ultimate tensile strength or yield point of such soft materials are in question, appropriate fixtures are sometimes available. These tests can and do accurately reflect certain properties of materials with greater or less accuracy.
In recent years, however, materials such as elastomers have begun to be used in other structural applications, i.e., in areas wherein the properties of stiffness in bending, for example, are of concern. Since the materials are soft, the absolute values are relatively low, and accuracy of measurement during testing is therefore of great importance.
A poorly designed test fixture is one which permits or encourages erratic or inaccurate readings. In measuring the stiffness of flexible materials of low modulus, available equipment has typically required that the material be inserted in a test fixture and tested in the same manner as harder, low deformation materials. For tests of harder materials, the test fixture usually included specimen support points or bars with sharp edges. While surface-hard materials are not affected by such fixtures, when softer materials are placed on such supports, surface engagement between fixture and specimen may cause grabbing or holding of the materials when bending stiffness is being measured. This introduces error into the bending stiffness measurement, either in absolute terms or in erratic, non-repeatable observations. The difficulty with proper observation is aggravated where the bending tests involve specimen deflection that is "seen" by the instrument as extension or tensioning of the specimen.
The present invention overcomes the aforementioned problems and difficulties in the prior art by providing an apparatus for measuring the stiffness of softer materials especially surface-soft materials, in bending. The apparatus incorporates a specialized anvil and yoke member for use with a conventional tensiometer. Both the yoke and the anvil are equipped with specimen-engaging cylindrical rollers at the point of specimen contact. Using this fixture, the required bending force supplied to the materials being tested can be accurately assessed. The cylindrical supports have a substantially circular cross-section, and are mounted relative to the yoke and the anvil using low-friction means such as ball bearings or bushing to allow the rollers to rotate freely about their longitudinal axes. In this manner, a material such as a rubber or other flexible material can be tested for stiffness in bending without undesirably introducing another force or term, i.e., tensioning of the specimen as a whole, to the measurement. As pointed out, if there is a problem of the support grabbing the specimen, it is almost certain that the specimen material will stretch or extend as it undergoes bending.
The rotatable supports eliminate the grabbing associated with prior art devices for such surface-soft materials, since rotation of the supports allows the sheet of material to advance over the supports under the load applied by the anvil and its associated loading nose. This provides a nearly frictionless surface to support the sheet of material while the bending stiffness measurement is being taken. In the preferred embodiment, the apparatus is provided as a three point flexural attachment to a conventional tensiometer wherein all three specimen-engaging elements are cylindrical rollers.
In view of the failure of the prior art to provide a fully satisfactory test fixture, it is an object of the invention to provide an improved test fixture apparatus for the measurement of bending stiffness characterizing certain materials with soft surfaces.
Another object of the invention is to provide a test fixture that provides improved accuracy when measuring the stiffness of specimens of certain surface-soft materials under an applied bending movement.
A further object of the present invention is to provide a test fixture for measuring stiffness of certain soft materials wherein the test fixture includes low friction, cylindrical supports to allow for test specimens to ride over the support rather than being grabbed by the support so as to create false or erratic readings.
A still further object of the invention is to provide a test fixture which is adaptable for use with existing tensiometers or other test devices.
A further object of the invention is to provide a low-cost test apparatus which is easy to manufacture and which is reliable in use.
Another object of the invention is to provide a test fixture apparatus for determining the bending stiffness of a specimen having a soft surface wherein the forces imparted to the specimen are transmitted through cylindrical elements that are free to rotate about their own axes so as to minimize or eliminate the effect of frictional contact between the surface of the specimen and a supporting surface of the test fixture.
Another object of the invention is to provide a test fixture apparatus which retains its advantages even if made in different sizes to accommodate test specimens.
Yet another object of the invention is to provide a simple and inexpensive test fixture using rotatable contacts or supports for a specimen and which is capable of determining bending stiffness and other properties of test specimens of a wide variety of materials having soft exterior surfaces.
Another object of the invention is to provide a test fixture which is self-centering in operation and requires little, if any, calibration for effective use.
The foregoing and other objects and advantages are achieved by providing a test fixture for measuring the bending stiffness of a specimen of flexible material having soft exterior surfaces, the test member including a yoke member and an anvil member each including legs arranged in pairs and adapted to support rotatable contact elements which, in position of use, respectively support a test specimen from below and engage it from above so as to impart a measurable bending movement to the specimen as the anvil and yoke move relative to each other.
The manner in which the foregoing and other objects and advantages are achieved in practice will become more clearly apparent when reference is made to the following detailed description of the preferred embodiments of the invention set forth by way of example and shown in the accompanying drawings, wherein like reference numbers indicate corresponding parts throughout.